Documentation

The CBMgr (Callback manager) encapsulates (in an enclosure, no less!)
an IORef VPUI. It is used *solely* to set up callbacks
and similar stuff in Gtk, where the callback needs access
to the IORef. By passing a CBMgr to a function, we can
avoid passing the IORef directly, and all the harm and
confusion that could result.

We only need *one* CBMgr for the application;
however, two CBMgrs with the same IORef are logically equivalent,
so there would be no harm in having two as long as they share one IORef.